The present invention relates to the art of nuclear magnetic resonance. The present invention finds particular application in the radio frequency (RF) coils used on a nuclear magnetic resonance medical imagers (MRI). The present invention may also find application in other magnetic resonance applications, such as well logging, chemical analysis, and the like.
Heretofore, whole body and localized receiving coils have been used for magnetic resonance imaging and spectroscopy of selected body areas of the patient. First, a standard sized whole body coil or body portion receiving coil is dimensioned to be disposed around the portion of the patient to be imaged. Due to the standard sizing, a significant void or empty region is defined between the coil and the portion of the patient to be imaged. As the imaged portion of the patient becomes a smaller fraction of the coil volume, the signal-to-noise ratio is decreased, degrading the image quality. One type of surface or localized coils is formed by mounting foil, wire or other conductors on stiff or rigid flat sheets of plastic or nylon. The flat coils have been constructed in a variety of sizes to facilitate being positioned adjacent a selected area of the patient to be imaged. However, their planar nature permitted only limited, partial contact with the patient.
The standard sized whole body or body portion coils and the rigid, planar coils did not permit optimization of image quality. Rather, the lack of conformity with the patient failed to optimize the filling factor and decreased the signal-to-noise ratio. These rigid coils received resonance signals from over a significantly larger area than the region of interest. This sensitivity to extraneous information degraded spatial resolution and increased aliasing in two and three-dimensional Fourier transform techniques. Improvement in the homogeneity of receiver sensitivity across the imaged space sacrificed the quality or Q factor of the coil, particularly in coils having resonance frequencies above 20 MHz. Moreover, the rigid coils were difficult to apply to the patient, uncomfortable, and created a need for a large range of inventory of coil sizes.
Other problems have been encountered in transferring the signal received by the high impedance coil over low impedance transmission lines to a remote, out of the image region preamplifier. To minimize signal loss and noise, the length of the low impedance transmission line was minimized. Although short unmatched transmission lines functioned acceptably at low frequencies, the Q factor of the coil degraded rapidly with increasing frequency and cable length.
Matching the transmission line length to the wavelength at the operating frequencies resulted in excessive length at low and mid-field strengths and lines that were too short at high fields. Because the coil impedance greatly exceeded the transmission line impedance, high cable losses attributable to standing waves on the cable were experienced. Moreover, non-zero cable dielectric and conductor losses damped the surface coil.
Attempting to match the impedance of the transmission line, whether balanced or unbalanced, has been unsuccessful. The normal variations in patient loading caused a corresponding impedance mismatch and resultant power transfer loss. At mid and high magnetic field strengths, the patient loading mismatch and transmission loss were magnified.
To overcome many of these disadvantages, flexible coil products have been manufactured from soft copper tubing and covered with a urethane foam. U.S. Pat. No. 4,920,318 of Misic, et al. discloses a flexible coil design with integrated preamplifiers. These products however, suffered from other disadvantages and limitations.
First, the copper tubing was difficult to flex and exhibited problems with stress hardening and embrittling. Also, the prior copper tubing coils were simple, linear coils which were not amenable to complex coil geometries. Furthermore, they provided limited patient comfort.
The present invention provides a new and improved coil and coil signal transmission system which overcomes the above-referenced problems and others.